1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus in which a plurality of developing means is arranged around a single image carrier for selectively developing a latent image formed on the image carrier.
2. Description of the Background Art
An image forming apparatus of the type using an image forming unit in which two developing devices are arranged around a single photoconductive element is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-177286, 11-44982, 22-109708 and 2000-242058 and Japanese Patent Application No. 2000-371438. In this type of image forming apparatus, a plurality of image forming units, each including two developing devices, each develop latent images with the respective developing devices. The resulting toner images are sequentially transferred to a single intermediate image transfer body one above the other, completing a color image.
In each image forming unit, one of the two developing devices is held operative for development while the other developing device is held inoperative, so that two toner images of different colors are sequentially formed on the photoconductive element. For example, to form a full- or four-color image, an image forming apparatus including two image forming units is operated such that toner images of two different colors are formed on each of two photoconductive elements one after the other while being sequentially transferred to an intermediate image transfer belt one above the other.
However, the image forming apparatus of the type using two image forming units is likely to wear and deteriorate the photoconductive elements and developers. More specifically, to produce a color image, latent images are sequentially formed on each photoconductive element and developed by the developing devices of each image forming unit. On the other hand, when a black-and-white image is to be produced, image formation is not executed with the photoconductive element associated with one image forming unit, which does not include a black developing device, although image formation is executed with the other image forming unit including the black developing device. In a black-and-white mode, therefore, the image forming unit not including the black developing device may be caused to stop operating, i.e., two developing rollers thereof both may be caused to stop rotating. However, when the developing rollers are brought into a halt with developers deposited thereon contacting the photoconductive element, toner contained in the developers rub the surface of the photoconductive element, accelerating the wear and deterioration of a photoconductive layer formed on the surface of the element.
To solve the above problem, the developing rollers of the image forming unit not performing image formation may be continuously rotated. This, however, brings about another problem that paddle rollers, screw conveyors and other agitating members, operatively connected to the developing rollers, are also driven, accelerating the wear and deterioration of the developers.
It is therefore necessary to space, in the image forming unit not performing image formation, both of the developers deposited on two developing rollers from the photoconductive element. Various methods have heretofore been proposed for preventing the developer on the developing roller of one inoperative developing device, as distinguished from the other or operative developing device, from contacting the photoconductive element. For example, Laid-Open Publication Nos. 11-44982 and 11-109708 mentioned earlier each propose a method and a configuration for reversing the direction of rotation of the developing roller to thereby remove the developer from the developing roller when development is not under way. Laid-Open Publication No. 2000-242058 teaches a method and a configuration for, by using non-contact development, constantly spacing the developers on the developing rollers from the photoconductive element while ON/OFF controlling a bias for development. Further, Japanese Patent Laid-Open publication No. 11-338257 proposes to locate a sleeve and a magnet, which is rotatable about the axis of the sleeve, upstream of the developing position of a developing roller and causes the magnet to rotate to selectively interrupt the feed of a developer to the developing position. Such conventional schemes, however, each have a particular problem left unsolved, as will be described hereinafter.
The reverse rotation scheme taught in Laid-Open Publication Nos. 11-44982 and 11-109708 is not practicable unless the developing roller is rotated for some period of time in order to remove the developer present thereon. During such a period of time, no toner images can be formed on the photoconductive element.
As for the non-contact development scheme disclosed in Laid-Open Publication No. 2000-242058, while the developer and photoconductive element perform non-contact development, the distance between the photoconductive element and the developing roller, i.e., a gap for development should preferably be small enough to enhance image quality. For this reason, a sufficient distance is not available between the developer and the photoconductive element. If development is effected in such a condition without using means for removing the developer from the developing roller, then toner deposits on the photoconductive element at the boundary between the exposed portion and the non-exposed portion of a latent image. As a result, color mixture occurs in the developing device or on the photoconductive element, lowering image quality. This kind of toner deposition is derived from a so-called edge effect ascribable to the enhancement of an electric field around the boundary between the image and non-image portions.
The selective feed scheme proposed in Laid-Open Publication No. 11-338257 requires the developing roller to rotate for some period of time in order to remove the developer on the developing roller. This not only obstructs high-speed operation, but also makes the apparatus sophisticated and bulky because a mechanism for implementing selective feed is essential.
In the circumstances described above, the surest way to prevent the developers on the developing rollers from contacting the photoconductive element may be retracting the developing devices from the respective developing positions when not effecting development. However, because one of the two developing devices is sometimes is used, the above scheme is not practicable unless each developing device is provided with a respective member supporting it in a retractable manner and a respective drive mechanism and a respective space for movement, which make the apparatus sophisticated, bulky and high cost.
A black-and-white mode is not the only case that requires both of two developing devices assigned to a single photoconductive element to be moved to their non-developing positions. For example, even when the apparatus is performing operation other than image formation, it is sometimes necessary to drive a member for cleaning an intermediate image transfer belt and members for cleaning the photoconductive drums. In this case, too, if the developers on the developing rollers are held in contact with the photoconductive element, then toner reduces the life of the photoconductive element, as stated earlier. Therefore, the above requirement should be met not only by the apparatus of the type including a plurality of image forming units each including two developing devices for a single photoconductive element, but also by an apparatus of the type including two developing devices for a single photoconductive element.
Japanese Patent Application No. 2001-371438 proposes an image forming apparatus in which two developing devices, included in an image forming unit, are constructed into a single developing unit angularly movable to switch the developing devices as to development/non-development. Although the developing devices of this kind of apparatus can share a support member, a drive mechanism and a space for movement, the developing devices cannot be moved to their non-developing positions at the same time.
Further, Japanese Patent Laid-Open Publication No. 11-125968 discloses an apparatus and a method for image formation configured to provide a period of time necessary for the switching of developing devices with a certain width for thereby providing switching means with a width of selection. This apparatus also has a problem that a sophisticated, bulky mechanism is required for switching the developing devices, resulting in an increase in size and cost.